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Re: [EnergyPlus_Support] Re: A few problems with controls
Sorry about the delay in answering your followup questions.
1. By dummy zone, I mean an extra zone that has a thermostat which controls it
to the OA supply air temperature, 20C in this case. This zone becomes a source
for the MIXING object to push a specified flow of air at 20C into the real
zones.
2. MIXING simplies transfers a certain flow rate of air from one zone to
another. The receiving zone assumes that an equal quantity of air leaves and
so it has a gain or loss based on TsourceZone - TreceivingZone. The source
zone know nothing about the MIXING process. It is up to the user to provide
makeup air to the source zone which balances the flow sent to the other
zone(s). If you use this approach just for sizing, then it doesn't really
matter, it's just a way to offset some load in the PTAC zones.
3. Connecting the PTAC outlet to a mixer will not work.
4. Another approach might be to use the dummy zone for your final simulations
as well. Redirect the 100% OA air loop to supply flow to the dummy zone with a
direct air unit. Then use MIXING objects to distribute this flow into the PTAC
zones. With the air loop servince the dummy zone, and the PTACs serving
different zones, you now have the possibility of getting autosizing to work
correctly, because you have access to different ZONE and SYSTEM SIZING objects
for the two system types. I have not tried this, so there may be a hitch, but
it could work.
Mike
On 13 Dec 2006 at 18:40, asheaffe wrote:
> Mike,
>
> Thanks for your help. I have a couple questions regarding your
> response. When you say that I should set up a dummy zone, do you
> mean to set up a dummy zone in addition to the existing zone? Or
> set up a simulation with a dummy zone in place of the existing zone?
>
> Also, can you explain a little more about how to use the mixing
> objects?
>
> Lastly, would a viable option be to have the PTAC output spit into a
> mixer which mixes the existing air and then outputs to the zones?
> Or maybe I should for go the PTAC and just set up a parallel loop to
> the constant volume OA ventilation system that takes in return air
> and outputs to the zone input?
>
> I put the other cooling coil in and it worked like a charm.
>
> Thanks again,
>
> Andrew
>
>
>
> --- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "Michael J. Witte"
> <mjwitte@...> wrote:
> >
> > I do not see a way to autosize this type of system.
> >
> > 1. Regarding the sizing of the PTACs. EnergyPlus autosizing does
> not know how
> > to deal with more than one system at a time. So, you will not be
> able to
> > autosize both of these. The autosizing also does not know about
> the
> > contributions from other systems, so there is no straightforward
> way to
> > autosize the PTAC units. The best way I can think of to get the
> values for
> > these PTAC units would be to create a dummy zone that is
> controlled to 20C with
> > purchased air and use MIXING objects to push the desired volume of
> 20C air into
> > the affected zones. Run this with the PTACs autosized, and then
> take the
> > values from the eio outputs and put them back in as hard sizes.
> Tedious, I
> > know.
> >
> > 2. Regarding control of the OA supply DX cooling coil. Your
> controls are
> > correct. The DX coil is not large enough to handle the load. If
> you report
> > "1E DX OUTLET:System Node Setpoint Temp[C]" and "1E DX:DXSystem
> Part-Load
> > Ratio" you will see that the DX coil is running at 100% most of
> the cooling
> > design day, yet it is unable to satisfy the setpoint. There are a
> few hours in
> > the morning where the setpoint is satisfied (03:00 to 06:30). Why
> is the coil
> > undersized? Autosizing of DX coils is limited to keep the coil
> within the
> > models limits of 300 to 450 cfm/ton (see Engineering Ref. pp. 298-
> 299, pdf pp.
> > 332-333). If you look at the autosized outputs in the eio file,
> the ratio of
> > flow/capacity is 4.026963e-5 m3/s/W which is 300 cfm/ton.
> >
> > The reality here is that a DX coil capable of treating 100%
> outside air in
> > Chicago to 12.8C is going to run at less than 300 cfm/ton which
> can then have
> > problems with freezing up the coil. It is likely that the actual
> equipment to
> > serve such a load will have two or more DX coils in series. You
> can model this
> > easily in EnergyPlus by DXSystem:AirLoop objects in series. The
> second one
> > should run only when the first one cannot meet the load. Of
> course, the
> > EnergyPlus autosizing will size them both to be 300 cfm/ton of
> system flow
> > rate. You will probably need to hand size these as well.
> >
> > Mike
> >
> >
> >
> > On 12 Dec 2006 at 0:58, asheaffe wrote:
> >
> > > Boy am I confused. I have several problems with the 1EOnly
> file.
> > >
> > > I want to have a ventilation system that supplies 100% OA. The
> > > ventilation systems needs to have a constant 12.8 C supply temp
> from
> > > the cooling coil, and a constant 20 C supply temp from the
> heating
> > > coil, at a constant flow rate of 0.092m3/s. Then I want a PTAC
> to
> > > take over the supplemental heating and cooling loads throughout
> the
> > > year (no OA flow through the PTAC, zone recirculation only).
> > >
> > > I can't seem to get the cooling coil to supply a constant 12.8,
> > > regardless of the setpoint manager that I have installed.
> > >
> > > Then, when I specify a flow rate for that main system, it is
> > > automatically applied (against my wishes) to the PTAC. How do I
> > > make it so the flow through the PTAC is autosized based on the
> > > supplemental heating and cooling needs?
> > >
> > > Also, for a system like this where I have two different flows
> being
> > > supplied at different temperatures to the room, what should I
> use
> > > for the heating and cooling design temp fields in the zone
> sizing?
> > >
> > > Thanks,
> > >
> > > Andrew
> > >
> > >
> > >
> > > The primary EnergyPlus web site is found at:
> > > http://www.energyplus.gov
> > >
> > > The group web site is:
> > > http://groups.yahoo.com/group/EnergyPlus_Support/
> > >
> > > Attachments are not allowed -- please post any files to the
> appropriate folder
> > > in the Files area of the Support Web Site.
> > >
> > > EnergyPlus Documentation is searchable. Open EPlusMainMenu.idf
> and press the
> > > "search" button.
> > >
> > > Yahoo! Groups Links
> > >
> > >
> > >
> > >
> > >
> >
> >
> > ========================================================
> > Michael J. Witte, GARD Analytics, Inc.
> > EnergyPlus Testing and Support
> > EnergyPlus-Support@...
> >
>
>
>
>
> The primary EnergyPlus web site is found at:
> http://www.energyplus.gov
>
> The group web site is:
> http://groups.yahoo.com/group/EnergyPlus_Support/
>
> Attachments are not allowed -- please post any files to the appropriate folder in the Files area of the Support Web Site.
>
> EnergyPlus Documentation is searchable. Open EPlusMainMenu.idf and press the "search" button.
>
> Yahoo! Groups Links
>
>
>
>
>
The primary EnergyPlus web site is found at:
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